Basically any IBM FRU drive that is less than 100GB will work. With that said, there are specific FRU's I'd look for;
(I made an EBay search with all the below FRU numbers when I was looking for one).
Code:
Hard drive
30GB, 9.5 mm, 4,200 rpm
* Supported by CTO
* 2668-CTO
* 2669-CTO
Fujitsu 13N6903 1
HGST 13N6883
=========================
Hard drive
40GB, 9.5 mm, 4,200 rpm
* Supported by CTO
* 2668-CTO
* 2669-CTO
Fujitsu 13N6905 1
HGST 13N6885
=========================
Hard drive
40GB, 9.5 mm, 5,400 rpm
* 1871-CTO, 11x, 12x, 13x, 14x, 15x, 16x, 17x, 18x, 1Gx, 1Hx, 22x, 23x, 24x, 48x, 4Ax, 4Bx, 4Cx, 86x, C1x, F1x, F4x
* 1872-CTO, 12x, 4Ax, C1x
* 1873-4Ax
* 1874-4Ax
* 1875-CTO, D1x, D2x, D3x, D6x, D9x, DAx, DCx, DKx, F1x, F4x
* 1876-CTO, DBx
* 2668-CTO, 1Bx, 1Cx, 1Dx, 1Fx, 21x, 28x, 29x, 3Fx, 3Gx, 3Hx, 3Jx, 3Nx, 41x, 42x, 43x, 44x, 45x, 46x, 47x, 49x, 4Dx, 4Ex, 4Jx, 56x, 57x, 61x, 67x, 68x, 69x, 6Ax, 6Bx, 6Dx, 6Ex, 6Jx, 81x, 82x, 83x, 84x, 85x, 87x, 88x, 8Dx, K3x, K4x, K5x, K6x, K7x
* 2669-CTO, 42x, 43x, 44x, 46x, 4Dx, 61x, 68x, 6Ax, 6Ex, 83x, 84x, C7x, C9x, CQx, CSx, K6x, K7x
* 2678-42x
* 2679-42x
* 2687-D4x, D7x, DEx
Fujitsu 13N6913 1
39T2549
HGST 39T2521
Toshiba 13N6919
===============================
Hard drive
60GB, 9.5 mm, 5,400 rpm
* 1871-CTO, 19x
* 1875-CTO, DLx, DMx
* 2668-CTO, 1Ax, 3Lx, 3Px, 4Fx, 4Gx, 4Kx, 4Mx, 4Nx, 6Cx, 6Fx, 6Hx, 6Kx, 6Mx, 6Px, 6Qx, 6Ux, 6Vx, 6Wx, 6Yx, 8Ax, 8Bx, 8Ex, C3x, C5x, CKx, CNx, F2x, F5x, F9x, FLx, FNx, K8x, KCx, KDx, KEx, KJx
* 2669-CTO, C3x, C5x, CKx, CNx
* 2686-DFx, DHx, F2x, F5x, FLx, FNx, M4x, M5x
* 2687-D3x, D8x, DCx, DDx
Fujitsu 13N6915 1
39T2551
HGST 39T2523
================================
Hard drive
60GB, 9.5 mm, 7,200 rpm
* Supported by CTO
* 2668-CTO, 78x, 94x, 98x, F8x, G1x, G2x, G3x, G4x, G5x, G6x, G7x, G8x, H1x, H2x, H3x, H4x, H5x, H7x, H8x, HAx, HBx
* 2669-CTO, H2x
* 2678-H2x
* 2679-H2x
* 2686-DGx, F8x
HGST 13N6897 1
===============================
Hard drive
80GB, 9.5 mm, 5,400 rpm
* 1871-CTO, 25x, 26x, 27x
* 2668-CTO, 1Ex, 2Ax, 3Kx, 3Mx, 4Hx, 4Lx, 4Qx, 58x, 6Gx, 6Lx, 6Nx, 6Xx, 71x, 72x, 73x, 74x, 75x, 76x, 77x, 89x, 8Cx, 91x, 92x, 93x, 95x, 96x, 97x, 99x, 9Ax, C2x, C4x, C6x, CLx, CMx, CPx, F3x, F6x, F7x, FMx, FPx, FQx, G9x, GAx, GBx, H6x, H9x, K9x, KAx, KFx, KHx
* 2669-CTO, C2x, C4x, C6x, C8x, CAx, CLx, CMx, CPx, CRx, CTx
* 2686-DJx, F3x, F6x, F7x, FMx, FPx, FQx, M7x
* 2687-D5x
Fujitsu 13N6917 1
39T2553
Toshiba 39T2557
HGST 39T2525
================================
Hard drive
80GB, 9.5 mm, 7,200 rpm
* 2668-FRx, KBx, KKx, J1x, J2x, J4x, J8x, L3x, L5x, P1x, P3x, P4x, PCx, Q1x, Q3x, Q6x, Q7x
* 2669-P1x, Q1x
* 2686-FRx
HGST 39T2583 1
=================================
Hard drive
100GB, 9.5 mm, 5,400 rpm
* KGx, J3x, J9x, L1x, L2x, L4x, L6x, L7x, L8x, L9x, LAx, P9x, PAx, PBx
Fujitsu 39T2555 1
Toshiba 39T2559
==================================
Hard drive
100GB, 9.5 mm, 7,200 rpm
* 2668-P2x, P5x, P6x, P7x, P8x, Q2x, Q4x, Q5x, Q8x
HGST 39T2585